The current view about the Pantheon’s construction history
In 27 BC Agrippa built the Pantheon. His name M·AGRIPPA can still de read on the architrave. “Or in full, “M[arcus] Agrippa L[ucii] f[ilius] co[n]s[ul] tertium fecit,” meaning “Marcus Agrippa, son of Lucius, made [this building] when consul for the third time.”
“In 202, the building was repaired by the joint emperors Septimius Severus and his son Caracalla (fully Marcus Aurelius Antoninus), for which there is another, smaller inscription on the architrave of the façade, under the aforementioned larger text. This now-barely legible inscription reads:
IMP · CAES · L · SEPTIMIVS · SEVERVS · PIVS · PERTINAX · ARABICVS · ADIABENICVS · PARTHICVS · MAXIMVS · PONTIF · MAX · TRIB · POTEST · X · IMP · XI · COS · III · P · P · PROCOS ET
IMP · CAES · M · AVRELIVS · ANTONINVS · PIVS · FELIX · AVG · TRIB · POTEST · V · COS · PROCOS · PANTHEVM · VETVSTATE · CORRVPTVM · CVM · OMNI · CVLTV · RESTITVERVNT
In English, this means:
Emp[eror] Caes[ar] L[ucius] Septimius Severus Pius Pertinax, victorious in Arabia, victor of Adiabene, the greatest victor in Parthia, Pontif[ex] Max[imus], 10 times tribune, 11 times proclaimed emperor, three times consul, P[ater] P[atriae], proconsul, andEmp[eror] Caes[ar] M[arcus] Aurelius Antoninus Pius Felix Aug[ustus], five times tribune, consul, proconsul, have carefully restored the Pantheon ruined by age.” Source: Wikipedia
The old temple was built in 27 BC and after a fire rebuilt by Domitian in 80 CE. The building was substantially damaged by a fire again in 110 C.E., the Emperor Trajan decided te rebuild it. Only much later, in 1892, did research by the French architect Georges Chedanne show that the Pantheon as it still exists today dates from 118-125 CE. Research of the foundations has led to the discovery of the dates 120 and 125 on many of the stones (they thought so until 2007). The old foundations from Agrippa’s time were partly reused. The temple that Hadrian designed is very different from its predecessor. Hadrian moved the entrance to the north side and built a round cella instead of the original square one.
Livius wrote that Agrippa’s temple was built on the site where Romulus ascended to heaven. A storm suddenly rose up during a meeting and everything was plunged into darkness. After the storm passed, those present to their utter amazement found themselves looking at an empty throne. Romulus had entered the world of the gods.
Even though Hadrian’s design was also inspired by emperor Nero’s golden house, it is nevertheless unique. Hadrian copied the alternation of round and square niches and the open oculus (literally: the eye) from an octagonal room in the golden house. Completely new is the round cella in a temple behind a square portico. As you have learned in class, neither the Greeks nor the Etruscans knew such a design.
Novel insights about the Pantheon’s construction history
“Emperor Hadrian had the Pantheon completely rebuilt between 119 and 125, giving it its current round shape” according to the Dutch Wikipedia. Archaeological research of the Pantheon’s foundation reveals that:
- The foundation of Agrippa’s temple was not rectangular like a classical Greek or Roman temple (Parthenon), but round (see layout: red marks the foundation of Agrippa’s temple and black Hadrian, Rodolfo Lanciani 1897).
- Agrippa’s temple had a portico with high columns, a pediment and a roundabout with the same dimensions as the current Pantheon and the entrance was oriented towards the North.
- The stamps in the bricks of the foundation are nearly all from the period when Trajan was emperor (110 to 117). The English Wikipedia does mention some of the new insights.
Dr. Paul A. Ranogajec, The Pantheon Khan Academy, Lise M. Hetland, “Dating the Pantheon,” Journal of Roman Archaeology 20 (2007), pp. 95-112 and Eugenio La Rocca, ‘Agrippa’s Pantheon and its origin in: Tod A. Marder/Mark Wilson Jones (ed), The Pantheon: From Antiquity to the Present, Cambridge University Press 2015 pp. 49-78. Carandini, A., with Carafa, P., (Edited by), The Atlas of Ancient Rome Biography and portraits of the City, (Volume) I and II, Princeton University Press, Princeton and Oxford, English translation 2017, (Volume) I p. 508 left column.
These three facts shed a new light on the current view. The attribution to Hadrian now has wavering and narrow support. After all, the foundation (for the most part) and the shape of the temple, are from the periods of Agrippa and Trajan. These details raise doubts.
Was the Pantheon constructed for the dynastic emperor worship, which started with August? The mausoleum of August was completed a few years prior to the Pantheon and is also situated at the Campus Martius, on the same axis as the Pantheon.
Who designed the Pantheon? Was it the architect of Trajan, namely Apollodorus of Damascus, who is also known for Trajan’s Forum? Did Hadrian simply want to do justice to Agrippa’s original temple with the inscription: M.AGRIPPA.L.F.COS.TERTIUM.FECIT.
Rodolfo Lanciani was right with his layout of Agrippa’s Pantheon (red marks the foundation of Agrippa’s temple and black Hadrian). Agrippa’s temple had a portico with high columns, a pediment and a roundabout with the same dimensions as the current Pantheon. But Agrippa’s Pantheon was clearly different in one respect: it could have had a concrete dome. In that time it was not possible to cover such a large space. The radius of the exedra which was about the space of the Pantheon was made of wood (Eugenio la Rocca, Agrippa’s Pantheon and its origin in: Tod A. Marder and Mark Wilson Jones (Edited by), The Pantheon From Antiquity to the Present, Cambridge University Press 2015 pp.64-66).
The facade of the portico
The front of the building features eight columns out of a total of 16, all carved out of a single block of pink and grey marble. The plinths and capitals were carved separately from precious Pentelian marble. The Corinthian columns were originally intended to be 15 meters high.
When one looks at the Pantheon’s facade from one of the sides, one can see the intermediate rectangular block with triangular tympan that seamlessly connects with the cornice of the cella. The triangular tympan of the portico does not properly connect with the cella.
“His [Agrippa] death, too, of which I shall speak next, and his deification after death, were known in advance by unmistakable signs. As he was bringing the lustrum to an end in the Campus Martius before a great throng of people, an eagle flew several times about him and then going across to the temple hard by, perched above the first letter of Agrippa’s name.” Suetonius, Augustus, 97
The pediment of the Pantheon had the image of an eagle.
How did this happen? The structure features no less than 16 monoliths (columns carved from a single block of marble) and it is a challenging task to carve out columns this big from a quarry and of exactly the right dimensions too. The Egyptian quarries where these columns were carved, could not fill this order in such a short period of time. They were, however, able to supply 16 twelve-meter columns. This is the reason why Hadrian used the twelve-meter columns and was forced to accept that the view of the portico was somewhat marred by the ugly connection with the cella. Fifteen-meter tall columns would have raised the portico to exactly the level of the second tympan of the intermediate block and connected beautifully with the cornice circling the rotunda (A. Claridge, Rome Oxford Archaeological Guides, Oxford University Press, Oxford 1998 page. 203)
In antiquity, one had to mount eight steps to reach the portico. These steps now lie buried under the sand that has piled up over the centuries
If you look at the building as a whole, it consists of three parts. The first part you see is the portico (a pronaos or entrance hall of a Greek temple). Next there is a transition to the naos or cella. This part, that forms the transition from pronaos to naos, has a barrel vault.
When you finally open the original bronze doors, you enter a big circular space: the naos, which is completely domed. One feature that immediately stands out is the big opening in the dome through which rain, wind, sun and moon can all enter freely. The diameter of this oculus is nine meters. The height from the ground is 43.3 meters, which also happens to be the exact diameter of this domed room. This means that you could inflate a perfectly spherical balloon with a diameter of 43.3 meters in this room, with the dome covering exactly half of this big balloon. Looking at the dome from the outside, the perfect sphere you saw on the inside is no longer discernible. This has to do with the construction of the Pantheon.
The construction of the Pantheon
The huge dome is nearly six meters thick where it rests on its piers but only 1.4 meters thick near the opening. The dome was made of concrete mixed with several kinds of crushed rock (for more information on Roman concrete click here). The materials used to build the dome become lighter and lighter as you near the top. The ring around the oculus is made of pumice, a very light kind of rock that contains much air.
The rotunda (without dome) can therefore be described as a series of pillars connected by walls or as two concentric walls connected by transverse walls. In modern terms, the rotunda is what we call a ‘diaphragm’; this structural element is relatively light and incredibly strong (source: Ziolkowski, A.,’Pantheon’ in Steinby, E.M. (et al), Lexicon Topographicum Urbis Romae (Rome 1999, p. 59 or click here for Drum and see footnote 33). The hollow recesses in the pillars or walls not only make it lighter, but also stronger.
Despite the clever us of various materials to make the dome as light as possible, there is still a huge amount of weight pressing down on the piers. The lateral forces generated by such a mass push the piers and the walls between them outward. This implies that the eight piers must be extremely stable to withstand the pressure. The lower ‘rings’ around the dome were built to keep the lateral forces under control. The eight piers, each pier six meters thick, that form part of the thick wall are made of concrete (click here; the gray color between the red is hollow). The construction above the eight niches in the cella was the cause of much amazement. How on earth is it possible that the two columns support an architrave without collapsing under the weight and the pressure of the dome? They should, according to the laws of physics. The reason why the two Corinthian columns are able to bear this enormous weight was hidden by the architect behind the marble and the coffers of the dome.
Hardly anything remains of the marble cladding on the outside. Today one can see the originally hidden load-bearing construction from the outside in the shape of round relieving arches. Relieving arches or rather barrelled vaults were built into the six-meter thick walls between the eight columns. The same technique [restoration of the coffers] was used for the dome. If one looks at the illustration above, one can see that these barrelled vaults channel the forces to the heavy piers instead of to the niches’ architraves. The forces that do press down on the architraves are channelled to the two Corinthian columns by smaller arches.
In antiquity the obelisk was often used as a sundial. That way you could more or less tell time and know where you were. The Pantheon not only tells each hour of the day and the night, but also what day it is. The current altar directly opposite the entrance always remains in shadow, while from the entrance, after passing through the heavy bronze doors, you could see the sun at its zenith. Standing in the cella between winter and summer, you could tell exactly what day of the year and what hour of the day it was. In this period, the sunlight would fall precisely on the projecting edge separating the wall from the dome and the first row of coffers. More information: Giulio Magli and Robert Hannah ‘The role of the sun in the Pantheon’s design and meaning’